The present invention relates to a plastic optical material, in particular for optical discs, lenses, fibers and glasses and a process for making the same.
Photopolymers are increasingly used in production of optical devices for all stages of optical discs manufacturing, such as replicate coatings for video discs and "direct read after write" discs, protective coatings for compact discs, substrates for "direct read after write" and erasable discs. They are also used for production of different kinds of lenses such as protective coatings and substrates for aspherical lenses, substrates and protective coatings for glass lenses, intraocular lenses, Frenel lenses. They are used for different technologies and stages of optical fibers production. Finally, they are used as adhesives and coatings for different kinds of multilayer glasses, etc.
It has been recognized that the properties of the main types of acrylic monomers and formulations on their basis do not assure the requirements presented to the most sophisticated parts of optical devices. It is especially difficult to achieve such necessary properties for optical photopolymers as high refractive index, low birefringence, low volume shrinkage, low moisture absorption, high flame retardance, the combination of sufficient impact and scratch resistance.
U.S. Pat. No. 4,477,328 discloses a photocurable resin composition on the basis of alkylene-bis (phenoxyalkyl-acrylate) represented by the formula ##STR3## However, the resulting substrates and coatings of optical discs and aspherical lenses are not-satisfactory as to moisture resistance, flame retardance, impact resistance, scratch resistance and further exhibits a considerable optical anisotropy (high birefringence).
EPA 0156372 A2 proposes to overcome the disadvantages of the prior art by using a photopolymerizable composition comprising
a compound having 4 or more (meth-)acrylic groups in one molecule, e.g. dipentaerythritolhexaacrylate;
a compound represented by the formula ##STR4## wherein R.sub.1, R.sub.2 =H.sub.1 CH.sub.3 ; ##STR5## or (CH.sub.2).sub.n (m=3 to 10); a compound of the formula ##STR6## wherein R.sub.4 is a hydrocarbon group having 6 to 16 C atoms, and a photoinitiator in an amount of 0.5 to 10 parts. However, according to this patent, the resulting article has too low a refractive index and a flame retardance and has a rather large volume shrinkage.
JP 4-372736 discloses an optical disc base made via a photosetting resin compound consisting of trimethylolpropane triacrylate, dipentaerythritol hexaacrylate modified with caprolactone, and hydroxypiperic acid neopentylglycol diacrylate modified with caprolactone. According to this application, the mixture of the aforesaid components in certain proportions is critical and provides a compromise between the unsatisfactory properties of each component, so that a Young modulus exceeding 100 kg/mm.sup.2 at room temperature is obtained together with satisfactory heat resistance and bending strength. No information, however, is provided as to the optical properties of the resulting polymer, particularly refractive index and birefringence.
In conclusion, the polymeric materials or photopolymerizable monomeric compositions of the prior art produce optical device bases which leave room for improvement as to the combination of mechanical and optical properties.